[python-package] fix mypy error about type change in Dataset.feature_num_bin()

[jameslamb]

Contributes to #3756.
Contributes to #3867.

mypy currently raises the following error.

python-package/lightgbm/basic.py:2491: error: Argument 1 to "c_int" has incompatible type "Union[int, str]"; expected "int"

This is caused by the fact that mypy doesn't allow changing the type of an object after it's initially assigned. This blog post explains it well: https://adamj.eu/tech/2021/05/23/python-type-hints-mypy-doesnt-allow-variables-to-change-type/.

This PR proposes a slight refactoring of Dataset.feature_num_bin() to resolve this error. I think this change does also make the code slightly easier to understand.

Notes for Reviewers

I tested this by running mypy as documented in #3867.

mypy \
    --exclude='python-package/compile/|python-package/build' \
    --ignore-missing-imports \
    python-package/

[github-actions]

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